A digital printing press is commonly used in the packaging and labelling industry for producing packaging items and labels. For certain merchandises, e.g., pharmaceuticals and food, there is a requirement to have zero defects on packaging items and labels, as printing errors can affect readability of sensitive information such as a warning statement and/or an ingredient list. Because the printing process itself is not perfect, zero defects can only be achieved by inspecting every printed instance and removing or correcting those with printing defects before sending printed instances to customers.
Because inspection by humans is slow, expensive and by no means guaranteed to catch all print defects, automated inspection systems are used in the industry. Such inspection system uses cameras to capture images of printed instances and processors to execute analysis algorithms on the captured images to check for presence of printing defects. This is done by comparing each captured image with a reference image of the packaging item or label. The reference image can be an image of a printed instance that is captured at the beginning of the print job and has been inspected by a human operator and found to be defect free. In more modern systems, the reference image is an image generated by a raster image processor (RIP) according to a page description language (PDL) that describes the graphics elements of the packaging item or label. Examples of the PDL are PostScript and Portable Document Format (PDF) by Adobe Systems, Inc., Mountain View, Calif.
The approach as described above works well when the packaging item or label is identical for all printed instances. More and more packaging items and labels, however, contain graphic or text elements that vary from instance to instance such as serial numbers, track and trace elements, and variable barcodes. The variable data makes each printed instance unique. The areas of the packaging item or label containing variable data are problematic for existing inspection systems. Since these areas are different for every printed instance, it is no longer possible to compare these areas to corresponding areas of the same reference image. Some inspection systems use Optical Character Recognition (OCR) techniques or barcode quality analysis algorithms for areas affected by variable text or variable barcodes to verify that the correct text or barcode is printed. These algorithms, however, are not able to detect printing defects such as spots, smears, and color shift.
An intuitive solution to the problem would be to have a reference image for every printed instance of the packaging item or label. Such a solution suggests a large set of reference images (one for each instance), which cannot be captured from the beginning of a printing job. In theory, the large set of reference images could be generated by using an RIP system to process a PDL file that represents the complete job of printing. A file in the ISO PDF/VT standard format, for example, can define one or more pages of which each page represents one printing target, e.g., a packaging label. Each page may contain multiple graphics elements including static graphics elements, which are identical for every printed instance of the page, and variable graphics elements, which are unique for every printed instance of the page. Using an RIP system, a set of reference images could be generated for every instance of a page and for every page of the printing job represented by the PDF/VT file. Each of the reference images would contain all static and variable graphics for one instance of a page. An inspection system could then compare a captured image of every printed instance to its corresponding reference image. Unfortunately, such a system is not practical as storage requirements (storing a complete unique image for every instance of a page) and bandwidth requirements (reading all the data for the complete unique image for every printed instance at the printing speed) are prohibitive.
Thus, in digital printing presses that deal with variable data, namely variable-data printing, there is a need for an inspection system and method that allow automatic inspection of printing defects on printed instances at the speed of printing.